In both radio and landline telephone systems, a user communicates by means of a handset that includes a speaker at one end that is placed close to the user's ear and a microphone at the other end that is held close to the user's mouth. This placement ensures that the audio from the speaker does not reach the microphone causing feedback problems. If the handset is taken away from the face, however, the speaker audio has a free path to the microphone. This is especially true when the handset is placed facedown on a surface. The surface acts as a reflector, reflecting the audio from the speaker to the microphone.
In addition, a microphone mounted directly to the mouthpiece in the handset, may cause feedback problems. The sound from the speaker causes the handset to vibrate and therefore the microphone, if not insulated from the handset, to vibrate. This vibration is transmitted by the microphone, causing the feedback problem to occur.
Government regulations restrict the amount of speaker audio to microphone coupling. Many countries have this type of regulation due to their cellular radiotelephone systems producing high sidetone levels. The higher sidetone levels would produce an irritating feedback howl if the speaker audio is coupled to the microphone. There is a resulting need for a way to isolate the microphone from the rest of the handset.